In this application, we will develop our finding that expression of the neurotrophin receptor trk B is inversely related to tumor progression and expression of the angiogenesis factor VEGF in medullary thyroid carcinoma (MTC). We have found that trk B is highly expressed in normal C-cells, precancerous hyperplastic C-cells, and microscopic MTC, but trk B expression is sharply diminished in later stages of MTC. in a cell culture model of human MTC, we have shown that introduction and expression of trk B results in severely impaired tumorigenicity in nude mice, accompanied by a marked reduction in VEGF expression. We hypothesize that trk B expression in MTC limits tumor growth by negative regulation of VEGF. This would suggest that VEGF may be an important therapeutic target in MTC. In addition, elucidation of the signal transduction pathways by which trk B downregulates VEGF may lead to new targets for control of VEGF production in other cancers. This application will explore these possibilities. 1. We will examine directly whether VEGF is necessary for tumorigenesis of MTC cells in nude mice. Thus, we will introduce antisense VEGF constructs into MTC cells, which basally express high levels of VEGF. We will also constitutively express VEGF in MTC cells in which we have expressed a trk B gene; these cells basally express low levels of VEGF and rarely form tumors in nude mice. Tumorigenicity will be monitored by subcutaneous injection into nude mice. Next, in a series of human MTC tumors, we will ask whether VEGF expression is inversely correlated with trk B expression. We will also examine whether other angiogenesis or growth related genes are also modulated by trk B in MTC cells. 2. We will investigate the mechanism of trk B control of VEGF expression, by examining VEGF transcription, RNA stability, and protein translation efficiency. 3. We will ask which signal transduction pathways are necessary fork B control of VEGF expression, by expression of mutant forms of trk B in MTC cells.